Someone buying an investment property doesn’t increase the amount of housing available. Sure, it’s one more rental available, but it’s also one less home available to be purchased by someone planning to live in it. I won’t claim to be any kind of expert, but it’s pretty obvious that it having a middle man extracting profit increases housing costs overall.
The type of investors that do increase housing are developers. In the tax model above, the developers can apply to have the tax reduced to 5% which seems to make it much more lucrative than buying individual houses to rent as is.